This invention relates to surface acoustic wave technology and in particular to a substrate comprised of a new cut of quartz having orthogonal temperature compensated propagation directions. The invention also relates to the application of substrates of that type to reflective array compressors and other surface acoustic wave reflective grating devices.
The use of multiple paths in surface acoustic wave devices is attractive because it provides good rejection of spurious signals and second order effects, however, it also places an additional condition on the substrate material if the device is to be temperature-stable without thermostatting. In the operation of a general reflection-grating device, the acoustic wave launched at an input interdigital transducer makes two 90.degree. reflections from the gratings and propagates along the orthogonal paths before reaching the output transducer. For temperature stability, it is necessary first that the substrate material be temperature compensated along the primary propagation axis; this is the usual requirement for temperature-stability of the dispersive delay. In addition, to avoid serious reductions in the amplitude of the output signal it is necessary that the temperature coefficients of delay along both propagation paths be equal. To meet both of these requirements, it is therefore necessary that the substrate be temperature-compensated along both propagation paths.
Since surface acoustic wave substrate materials are in general anisotropic, the requirement for temperature-compensated orthogonal propagation directions is not usually satisfied. On the popular ST-cut of quartz, for example, the temperature coefficient of delay along the direction orthogonal to the temperature-compensated X-axis is 47 ppm/.degree.c; hence, orthogonal-direction surface acoustic wave devices built on ST-cut quartz must be thermostatted. It appears, in fact, that no surface acoustic wave cut of any material has previously been found which is temperature-compensated along two orthogonal directions.
The present invention is directed toward filling that need.